This invention concerns the extractive separation of diglycolic acid from glycolic acid. In particular, diglycolic acid is extracted from aqueous mixtures of glycolic acid and diglycolic acid using an organic solution of a water-insoluble trihydrocarbylphosphine oxide.
U.S. Pat. No. 3,911,003, granted Oct. 7, 1975, describes the hydrogen fluoride-catalyzed reaction of aqueous formaldehyde and carbon monoxide to give glycolic acid. As described at Col. 3, the product is an aqueous mixture of glycolic acid and diglycolic acid. While the concentration of diglycolic acid can be minimized by maintaining a formaldehyde to water reactant ratio below about 4:1, some diglycolic acid is always produced. Fortunately, both acids are items of commerce, which when separated are readily marketable.
Unfortunately, glycolic acid and diglycolic acid are highly polar acids and, therefore, difficult to separate by either crystallization or distillation. As a consequence, where the acids are intended for use in the production of glycols, they are usually separated by esterifying the acids and hydrogenating the esters to form ethylene glycol and diethylene glycol, which can be separated by distillation. Where the acids themselves are desired, they are separated by distilling the esters, which can subsequently be hydrolyzed back to the acids. The major drawback to these separation procedures is the need to convert the acids to their corresponding esters prior to separation. It would be considerably more efficient to separate the acids as such.
Solvent extraction is one method of separation which has been used to separate highly polar organic compounds as such. Suitable extractants must have a preferential high solvent power for the component to be extracted. Thus, in the case of an aqueous mixture of glycolic acid and diglycolic acid, a suitable extractant must preferentially extract one or the other of the acids. The effectiveness of a two-phase extraction procedure depends upon the solvent power of the extractant and is measured by beta values. The beta value of a two-component extraction is defined by the expression: ##EQU1## In general, a beta value of at least 2 is required for an economical extraction process.
Accordingly, it would be advantageous to separate aqueous mixtures of diglycolic acid and glycolic acid using solvent extraction carried out with an extractant which provides a beta value in excess of 2 under typical operating conditions.